refactor: unify layout widths and font sizes

- Add MAX_CONTENT_WIDTH (820px) constant for consistent chat/form widths
- Bump theme font sizes (base now 16px) for better readability
- Apply max width to stream view, input area, and new agent form
- Position new agent form at bottom of scroll area
- Remove working directory helper text
- Add cmd+b sidebar toggle when textarea is focused

packages/app/src/constants/layout.ts

@@ -4,3 +4,6 @@ export const FOOTER_HEIGHT = 75;
 // Used by both agent header (ScreenHeader) and explorer sidebar header
 // This ensures both headers have the same visual height
 export const HEADER_INNER_HEIGHT = 48;
+
+// Max width for chat content (stream view, input area, new agent form)
+export const MAX_CONTENT_WIDTH = 820;